1. Field of the Invention
This invention relates to multipoint data communication systems of the kind including a master modem and a plurality of remote modems, wherein said master modem includes a modem receiver including sampling means adapted to receive analog signals from a transmission line and including timing recovery means adapted to supply timing signals to said sampling means.
2. Description of the Prior Art
A multipoint data modem communication system of the kind specified is known from European Patent Application No. 0,169,548. In the known system, a master station polls several remote stations. The modem of the master station gives a continuous carrier signal. This signal is received by each remote modem. If the terminal identity contained in the poll does not match the identity of the remote station then the poll is ignored. A remote station that recognizes the poll responds by sending one of two types of "messages"; either the first block of its data or a character sequence indicating no data to send. A remote station must use switched carrier signal when sending its "message" by turning on "request to send" to its modem before transmission and turning it off after transmission.
Once a particular remote station becomes active, the master station modem must detect the carrier of the remote modem, extract the receiver timing signal, set internal gain parameters, and acquire equalizer values to compensate for the transmission path to the active modem.
Each modem communication channel has a separate and distinct path resulting in a different set of characteristics corresponding to attenuation, amplitude and delay distortion and phase impairments. Therefore the master station modem must compensate for each communication channel individually. Each remote-master communication channel requires the connected modem to send a training sequence to allow the master modem receiver to acquire the parameters required for reliable data transfer. For that purpose, the master modem acquires receiver operating parameters and equalizer coefficients during an initial training sequence, and stores the parameters and coefficients in a memory location allocated for the particular remote modem. The stored parameters include parameters for compensation of data timing clock offset and equalizer coefficients. At the beginning of a subsequent transmission the master modem receiver parameters and coefficients are loaded from values previously stored in the corresponding memory location. During subsequent transmissions from a remote modem, this modem is identified at the master modem by a time interval or by a set of frequencies. During short remote-master transmissions each training sequence occupies a significant part of the transmission time. During training for transmission at high data-rates such as 14,400 bits per second the receiver operating parameters have to achieve accurate settings to yield a high data reliability.